The invention relates generally to an antenna, and more specifically to a dual polarized antenna for use in a cellular base station.
In traditional cellular base stations, vertically polarized antennas are used. These antennas often have a wide antenna beam in azimuth, e.g. 120.degree.. To increase the performance of the base station, two antennas may be mounted at a sufficient distance from each other to enable space diversity.
A similar increase in performance can be obtained by instead using polarization diversity. Two different, e.g. mutually orthogonal, polarizations from one and the same antenna can be used as diversity channels.
In antennas that are to be used in connection with polarization diversity, it is important that the two polarizations have the same antenna patterns, and that each polarization vector maintains its direction for all azimuth angles.
In a vertically linear antenna array for sector coverage, the radiating elements can be of different types, e.g. microstrip patch elements or dipoles. Also, a ground plane can be included in the antenna array.
One purpose of this ground plane, is to form the desired radiation pattern. To get the desired beam width, the ground plane is at least partly surrounded by side walls. These side walls will at the same time strengthen the antenna structure mechanically and hold interior parts of the antenna in place.
However, due to the electromagnetic properties of the edge of such side walls, vertical and horizontal polarizations will be affected differently. With side walls along the sides of the radiating element or elements, the beam width of the vertically polarized signal will be different from the beam width of the horizontally polarized signal.
If another combination of two polarizations is used to obtain polarization diversity, the edge effects may cause differences between the two polarizations both in the beam patterns and in the polarization vector directions. The effect for e.g. a pair of +/-45.degree. polarization directions is typically a rotation of the polarization vector rather than a beam width difference.